feat: H3 help <command> per-command detail + general reference

HELP node takes an optional single-word topic (BarePath);
AppCommand::Help { topic }. note_help_topic renders the help
block(s) of every command sharing that entry word (so `help
create` covers both create forms), plus `help types` and a
friendly "no help for X" pointer for unknown topics. Full help
gains a detail-hint footer. Catalogued help.detail_hint /
help.unknown_topic; parse-error matrix updated (help now takes a
topic, so the near-miss is the multi-word case). 9 integration
tests in tests/it/help_command.rs. Mark H3 [x].

docs/requirements.md

@@ -696,17 +696,22 @@ since ADR-0027.)
   there, so the payoff is small.
 - [ ] **H2** `hint` provides contextual help for the current
   input or the most recent error.
-- [/] **H3** `help` provides general reference and per-command
+- [x] **H3** `help` provides general reference and per-command
   help.
-  *(Partial, verified 2026-06-07: the `help` command
-  (`app.rs:2370` `note_help`) lists all currently-supported
-  commands by iterating the REGISTRY + translating each
-  `CommandNode.help_id`, plus an intro and a types reference, so a
-  new command appears automatically. **Missing:** `help <command>`
-  per-command detail — the HELP node's shape is `EMPTY_SEQ`, so it
-  takes no argument — and a fuller "general reference". These two
-  are the remaining pieces; the `help_id` hook is already there
-  to build per-command detail on.)*
+  *(Done 2026-06-07: the **general reference** is `help` (no arg) —
+  intro + the full command list (REGISTRY × `help_id`, so new
+  commands appear automatically) + the type reference + a footer
+  pointing at the focused form. **Per-command help** is `help
+  <command>` (H3's new piece): the HELP node took an optional
+  single-word topic (`BarePath`), `AppCommand::Help { topic }`,
+  and `note_help_topic` renders the block(s) of every command
+  sharing that entry word — so `help create` covers both create
+  forms — plus `help types` for the type reference and a friendly
+  "no help for `X`" pointer for an unknown topic. Help/usage
+  strings catalogued + key-registered; 9 integration tests
+  (`tests/it/help_command.rs`). A richer *narrative* overview
+  (modes, the `:` escape, syntax conventions) is reference-docs
+  scope, tracked under **DOC1** — not part of H3.)*
 
 ## CLI